docs/attachments/LUCENE-8580/LUCENE-8580.patch

diff --git a/lucene/core/src/java/org/apache/lucene/index/SegmentMerger.java b/lucene/core/src/java/org/apache/lucene/index/SegmentMerger.java
index 6554cc59da..7df98c68cf 100644
--- a/lucene/core/src/java/org/apache/lucene/index/SegmentMerger.java
+++ b/lucene/core/src/java/org/apache/lucene/index/SegmentMerger.java
@@ -18,7 +18,11 @@ package org.apache.lucene.index;
 
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ForkJoinPool;
+import java.util.concurrent.ForkJoinTask;
 
 import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.codecs.DocValuesConsumer;
@@ -94,97 +98,114 @@ final class SegmentMerger {
    * @throws IOException if there is a low-level IO error
    */
   MergeState merge() throws IOException {
+    ForkJoinPool fjPool = ForkJoinPool.commonPool();
+
     if (!shouldMerge()) {
       throw new IllegalStateException("Merge would result in 0 document segment");
     }
     mergeFieldInfos();
-    long t0 = 0;
-    if (mergeState.infoStream.isEnabled("SM")) {
-      t0 = System.nanoTime();
-    }
-    int numMerged = mergeFields();
-    if (mergeState.infoStream.isEnabled("SM")) {
-      long t1 = System.nanoTime();
-      mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge stored fields [" + numMerged + " docs]");
-    }
-    assert numMerged == mergeState.segmentInfo.maxDoc(): "numMerged=" + numMerged + " vs mergeState.segmentInfo.maxDoc()=" + mergeState.segmentInfo.maxDoc();
+
 
     final SegmentWriteState segmentWriteState = new SegmentWriteState(mergeState.infoStream, directory, mergeState.segmentInfo,
-                                                                      mergeState.mergeFieldInfos, null, context);
+        mergeState.mergeFieldInfos, null, context);
     final SegmentReadState segmentReadState = new SegmentReadState(directory, mergeState.segmentInfo, mergeState.mergeFieldInfos,
-                                                                   IOContext.READ, segmentWriteState.segmentSuffix);
+        IOContext.READ, segmentWriteState.segmentSuffix);
 
-    if (mergeState.mergeFieldInfos.hasNorms()) {
-      if (mergeState.infoStream.isEnabled("SM")) {
-        t0 = System.nanoTime();
-      }
-      mergeNorms(segmentWriteState);
-      if (mergeState.infoStream.isEnabled("SM")) {
-        long t1 = System.nanoTime();
-        mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge norms [" + numMerged + " docs]");
-      }
-    }
+    ArrayList<ForkJoinTask<Void>> tasks = new ArrayList<>();
 
-    if (mergeState.infoStream.isEnabled("SM")) {
-      t0 = System.nanoTime();
-    }
-    try (NormsProducer norms = mergeState.mergeFieldInfos.hasNorms()
-        ? codec.normsFormat().normsProducer(segmentReadState)
-        : null) {
-      NormsProducer normsMergeInstance = null;
-      if (norms != null) {
-        // Use the merge instance in order to reuse the same IndexInput for all terms
-        normsMergeInstance = norms.getMergeInstance();
+    // norms
+    fjPool.submit(() -> {
+      long t0 = System.nanoTime();
+      if (mergeState.mergeFieldInfos.hasNorms()) {
+        mergeNorms(segmentWriteState);
+        if (mergeState.infoStream.isEnabled("SM")) {
+          long t1 = System.nanoTime();
+          mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge norms");
+        }
       }
-      mergeTerms(segmentWriteState, normsMergeInstance);
-    }
-    if (mergeState.infoStream.isEnabled("SM")) {
-      long t1 = System.nanoTime();
-      mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge postings [" + numMerged + " docs]");
-    }
+      return (Void) null;
+    }).join();
 
-    if (mergeState.infoStream.isEnabled("SM")) {
-      t0 = System.nanoTime();
-    }
-    if (mergeState.mergeFieldInfos.hasDocValues()) {
-      mergeDocValues(segmentWriteState);
-    }
-    if (mergeState.infoStream.isEnabled("SM")) {
-      long t1 = System.nanoTime();
-      mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge doc values [" + numMerged + " docs]");
+    // stored fields
+    {
+      tasks.add(fjPool.submit(() -> {
+        long t0 = System.nanoTime();
+        int numMerged = mergeFields();
+        if (mergeState.infoStream.isEnabled("SM")) {
+          long t1 = System.nanoTime();
+          mergeState.infoStream.message("SM", ((t1 - t0) / 1000000) + " msec to merge stored fields [" + numMerged + " docs]");
+        }
+        assert numMerged == mergeState.segmentInfo.maxDoc() : "numMerged=" + numMerged + " vs mergeState.segmentInfo.maxDoc()=" + mergeState.segmentInfo.maxDoc();
+        return (Void) null;
+      }));
     }
 
-    if (mergeState.infoStream.isEnabled("SM")) {
-      t0 = System.nanoTime();
-    }
-    if (mergeState.mergeFieldInfos.hasPointValues()) {
-      mergePoints(segmentWriteState);
-    }
-    if (mergeState.infoStream.isEnabled("SM")) {
-      long t1 = System.nanoTime();
-      mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge points [" + numMerged + " docs]");
-    }
+    tasks.add(fjPool.submit(() -> {
+      long t0 = System.nanoTime();
+      if (mergeState.mergeFieldInfos.hasNorms()) {
+        try (NormsProducer producer = codec.normsFormat().normsProducer(segmentReadState)) {
+          mergeTerms(segmentWriteState, producer.getMergeInstance());
+        }
+      } else {
+        mergeTerms(segmentWriteState, null);
+      }
+      if (mergeState.infoStream.isEnabled("SM")) {
+        long t1 = System.nanoTime();
+        mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge postings");
+      }
+      return (Void) null;
+    }));
 
-    if (mergeState.mergeFieldInfos.hasVectors()) {
+    tasks.add(fjPool.submit(() -> {
+      long t0 = System.nanoTime();
+      if (mergeState.mergeFieldInfos.hasDocValues()) {
+        mergeDocValues(segmentWriteState);
+      }
       if (mergeState.infoStream.isEnabled("SM")) {
-        t0 = System.nanoTime();
+        long t1 = System.nanoTime();
+        mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge doc values");
+      }
+      return (Void) null;
+    }));
+
+    tasks.add(fjPool.submit(() -> {
+      long t0 = System.nanoTime();
+      if (mergeState.mergeFieldInfos.hasPointValues()) {
+        mergePoints(segmentWriteState);
       }
-      numMerged = mergeVectors();
       if (mergeState.infoStream.isEnabled("SM")) {
         long t1 = System.nanoTime();
-        mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge vectors [" + numMerged + " docs]");
+        mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge points");
       }
-      assert numMerged == mergeState.segmentInfo.maxDoc();
+      return (Void) null;
+    }));
+
+    if (mergeState.mergeFieldInfos.hasVectors()) {
+      tasks.add(fjPool.submit(() -> {
+        long t0 = System.nanoTime();
+        long numMerged = mergeVectors();
+        if (mergeState.infoStream.isEnabled("SM")) {
+          long t1 = System.nanoTime();
+          mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to merge vectors [" + numMerged + " docs]");
+        }
+        assert numMerged == mergeState.segmentInfo.maxDoc();
+        return (Void) null;
+      }));
+    }
+
+    for (ForkJoinTask<?> t : tasks) {
+      t.join();
     }
-    
+
     // write the merged infos
+    long t0 = 0;
     if (mergeState.infoStream.isEnabled("SM")) {
       t0 = System.nanoTime();
     }
     codec.fieldInfosFormat().write(directory, mergeState.segmentInfo, "", mergeState.mergeFieldInfos, context);
     if (mergeState.infoStream.isEnabled("SM")) {
       long t1 = System.nanoTime();
-      mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to write field infos [" + numMerged + " docs]");
+      mergeState.infoStream.message("SM", ((t1-t0)/1000000) + " msec to write field infos");
     }
 
     return mergeState;